Bulge Testing Transparent Thin Films with Moir, Deflectometry

The problem that is addressed here is the measurement of the mechanical properties of very thin, transparent films using bulge tests. All existing techniques make use of reflection from the film surface, but they can be difficult or impossible to apply to very thin, transparent films. Consequently, a novel approach based on the formation of a lens structure and using transmitted light is developed. In this technique, the focal length of the lens structure formed by the bulged film and the pressurizing medium is determined by moir, deflectometry with a corrected governing equation. The resulting curvature of the bulge film is used in the stress analysis of the bulge-test. By combining circular and rectangular configurations, the Young's modulus and Poisson's ratio of a 3 mu m PET film were 4.65 +/- 0.11 GPa and 0.34 +/- 0.01, respectively. Consistent residual stresses were obtained from both configurations.